1. Field
Embodiments described herein relate generally to a differential amplifying circuit and a microphone/amplifier system.
2. Background Art
Microphone/amplifier systems are used in audio equipment. Such a conventional microphone/amplifier system includes a microphone, a differential amplifying circuit and an A/D converter, for example.
A coupling capacitor is connected between the microphone and the differential amplifying circuit. The coupling capacitor usually has a capacitance of several μF to several tens of μF in order to pass an audio band of 20 Hz to 20 kHz. The differential amplifying circuit has a gain of about 40 dB in a tablet application, for example. The input impedance (≈input resistance) is as high as about 20 kΩ.
With the conventional microphone/amplifier system, in order for the differential amplifying circuit to stably operate, the differential amplifying circuit has to operate with the input terminals thereof kept stable at a predetermined potential (=operating point). If the potential at the input terminals deviates from the predetermined potential, the differential amplifying circuit has to quickly recover from the state.